The present invention generally relates to the torque or power transmission art. More specifically, the present invention relates to the operation and construction of continuously variable transmissions (CVT's). The present invention particularly provides a clutch control system and technique for regulating the transfer of torque from an engine to a drive train through a CVT and a control clutch.
The art contains numerous examples of the operation and construction of CVT's, e.g., U.S. Pat. No. 4,522,086 and U.S. Pat. No. 4,458,318, entitled Control System for Continuously Variable Transmission, and Control Arrangement for a Variable Pulley Transmission, respectively. These patents, especially the former, generally describe the mechanics and controls for a CVT system utilizing two adjustable pulleys, each pulley having at least one sheave which is actually fixed and another sheave which is actually movable relative to the first sheave. A flexible belt of metal or elastomeric material intercouples the pulleys. The interior faces of the pulley sheaves are beveled or chamfered. Thus, as the axially displaceable sheave moves relative to the fixed sheave, the distance between the sheaves and thus the effective pulley diameter may be adjusted. The displaceable sheave includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter; as fluid is exhausted from the chamber, the pulley diameter changes in the opposite sense. Generally, the effective diameter of one pulley is adjusted in one direction as the effective diameter of the second pulley is varied in the opposite direction, thereby effecting a change in the drive ratio between the input shaft coupled to an input pulley and an output shaft coupled to an output pulley. The ratio changes continuously as the pulley diameters vary. Such transmissions frequently are referred to in the art as a continuously variable transmission, or CVT.
Through the years various refinements have gradually evolved the hydraulic control system which is used to pass fluid into the fluid holding chamber of each adjustable pulley. An example of such a hydraulic system is shown in U.S. Pat. No. 3,115,049--Moan. In that patent, control of the secondary pulley adjustable sheave regulates the belt tension, while a different circuit regulates fluid into and out of the primary sheave to regulate the transmission ratio. U.S. Pat. No. 4,152,947--van Deursen et al. also describes control of a CVT. In both systems, the line pressure of the fluid applied to hold the belt tension by pressurizing the secondary chamber is kept at a relatively high value. An improved control system subsequently was developed to reduce the mainline fluid pressures supplied to the secondary sheave chamber as a function of torque demand. This improved system is described and claimed in U.S. Pat. No. 4,522,086, entitled "Control System for Continuously Variable Transmission," issued June 11, 1985, assigned to the assignee of this application. Further work resulted in an improved control system which reduced the line pressure applied to the secondary chamber to a low, safe operating pressure and also provided a lower control pressure for other portions of a hydraulic control system. This system is described in an application titled "Hydraulic Control System for a Continuously Variable Transmission", Ser. No. 421,198, filed Sept. 22, 1982, which issued Dec. 15, 1987, as U.S. Pat. No. 4,712,453, and assigned to the assignee of this application.
Additional significant advances in CVT control systems have been described and claimed in another application also entitled "Hydraulic Control System for a Continuously Variable Transmission," Ser. No. 717,913, filed Mar. 29, 1985, which issued Jan. 12, 1988, as U.S. Pat. No. 4,718,308, and assigned to the assignee of this application.
Another U.S. Pat. No. 4,648,496, issued Mar. 10, 1987, entitled "Clutch Control System for a Continuously Variable Transmission" further describes control logic techniques for regulating pressure at a clutch in CVT system to provide the desired torque transfer from an engine to a vehicle drive line. In that system, clutch control depends on logical recognition of one of a number of operating modes.
This control system is designed to handle those situations where the total vehicle torque, interia torque plus road load, will load the engine down. These situations occur when the vehicle is stationary or at a slow speed. For the other situations, when the vehicle torque would accelerate the engine rather than load it down, this normal start mode would yield an undesirable response. These other situations are controlled as set forth in a copending application entitled "Special Start Technique for Continuously Variable Transmission Clutch Control", Ser. No. 25,476, filed on an even date therewith.
The teachings of each of the above referenced patents and applications are incorporated herein by reference in terms of background to the present invention, although practical problems may arise in the implementation of the previously envisioned systems.